1. Field of the Invention
The present invention generally relates to packaging of electronic devices generally of a modular type including a plurality of chips and, more particularly, to such devices which are attached or formed integrally with structures including a compliant thermal material such as a thermal grease or gel for removing heat from the chips during operation.
2. Description of the Prior Art
Since the invention of the transistor, dissipation of heat during operation has been an important consideration in semiconductor device package design. Heat can damage the delicate and tiny structures which allow transistors to function as intended in respective designs. Power drawn by transistors, field effect transistors and other electronic devices, including the connections between them must be dissipated to avoid build up of heat and the development of high temperatures which can degrade the devices by such mechanisms as dopant diffusion and metal migration including solder softening and reflow.
As is known, a bipolar transistor ideally does not draw power during periods when it is quiescent or saturated but only while switching or modulating current therethrough. In practice, however, some power is drawn and must be dissipated at all times, even in digital switching and logic circuits which are primarily in either a quiescent or saturated state. However, power drawn by bipolar transistors increases rapidly with switching speed and clock rate which is emphasized in current data processing circuit designs.
Field effect transistors (FETs), which are becoming more generally used in digital data processing integrated circuits, especially integrated circuits formed at high integration density, exhibit a non-negligible channel resistance and, functioning as voltage-controlled variable resistors, draw some amount of power at all times during operation and in proportion to the current which they conduct. As an incident of integrated circuit design, however, the channel of a field effect transistor requires the principal portion of the chip area occupied by the transistor while the width and length of the channel determine the channel resistance. The length of the channel must be designed in view of the voltage at which the device is to operate; the reduction of operating voltage tending to compromise memory device function. Therefore, in modern logic and digital data processing circuits, the channel width is carefully engineered in view of loads on each transistor, balancing power dissipation against integration density.
The speed of operation and other operational characteristics such as noise immunity of integrated circuits can generally be enhanced by increased integration density since decreases in circuit element size (e.g. FET gates) and conductor length generally reduce drive current requirements of the electronic devices and reduce capacitive coupling between circuit elements, thus improving immunity to noise. Further, the number of devices which can be placed on a single chip generally increases manufacturing economy as well as reducing the cost and noise susceptibility of forming connections between chips. Therefore, in recent years, substantial emphasis has been placed on techniques and structures for removing heat from integrated circuit devices.
Heat transfer is impeded by both boundary conditions at surfaces of structures and the heat capacity (e.g. specific heat) and heat conductance properties of structures, including cooling fluids such as air which may circulate by natural convection and/or be mechanically circulated. For example, so-called thermal greases and gels have been developed and have been in use for many years for the purpose of forming intimate contact with both semiconductor packages, such as power transistors, and heat removal structures such as heat sinks and so-called cold plates which may have a cooling fluid circulated though passages therein in order to carry heat to remotely located heat transfer structures. High performance thermal materials in the form of a gel and suitable for practice of the present invention are disclosed in U.S. Pat. No. 4,852,646 to Dittmer et al. which is hereby fully incorporated by reference. Due to the intimacy of contact with both devices and the excellent heat conductance characteristics of the thermal grease or gel as well as the thinness of the grease when installed between the semiconductor package and the heat removal structure in many installations, the thermal grease can substantially increase heat transfer away from an electronic package. However, if intimacy of contact with a heat removal structure is not maintained over a significant area of the package, the interfaces with and thickness of the thermal grease may present an impedance to heat flux despite the relatively high thermal conductivity of currently available thermal greases and gels.
Accordingly, it must be recognized that the interface between the semiconductor chips on which the many transistors and other electronic devices are formed and a heat removal structure also impedes heat transfer. When many integrated circuit chips are mounted on a modular structure, the intimacy of thermal contact with a heat removal structure can only be achieved with difficulty and substantial process complexity. For example, U.S. Pat. No. 5,251,100 to Fujita et al. teaches an arrangement and methodology in which a chip surface arranging plate (CSAP) is bolted to a cooling arrangement and chips are connected to the CSAP with solder after applying a vacuum to attract the chip surfaces to the CSAP to establish a surface defined by surfaces of all chips which conforms to the CSAP. Interstices between chips and the module providing inter-chip connections and a heat removal structure forming a "cap" or "hat" for the package are then filled with thermal compounds such as thermal greases or low melting point solder. However, in so doing, metallization of surfaces of the chips and other surfaces to be bonded is required. The CSAP sits on the chip; hence, the chip area in contact with the thermal grease or gel is reduced. Additionally, the use of a conductive material such a solder can affect the functionality of the chips if required processes are not carried out perfectly.
U.S. Pat. No. 5,276,586 to Hatsuda et al. provides ceramic heat conduction members on respective chips to conduct heat to the module cap and includes a reservoir with an affinity for a bonding agent such as solder to receive excess amounts thereof. As with Fujita et al., described above, mislocation of conductive solder can affect chip functionality and decrease manufacturing yield, particularly if solder bonding agents spill over from one chip site to another if the intended effects of the reservoir are inadequate to prevent such spillage. Some other heat conduction structures are disclosed in U.S. Pat. No. 5,022,462 which uses a flexible sheet of thermally conductive material with upstanding fins and U.S. Pat. No. 5,052,481 which includes a thermally conductive oil and internal fins.
In summary, while many arrangements and structures are known for removing heat from semiconductor devices, simpler arrangements have marginal performance and are of inadequate thermal performance for dissipation of power from very high density integrated circuits, particularly when operated at high clock or switching rates. Known arrangements and structures having higher thermal performance generally involve complex structures and process requirements which increase cost and can reduce manufacturing yield. Generally, these latter arrangements and structures are only marginally adequate, at best, for removal of heat from state-of-the-art digital data processing circuit modules despite their high economic cost.
A simple yet effective arrangement for heat removal from electronic devices is done by so-called flat plate cooling (FPC) in which a flat plate is placed as closely as possible to the chip and the gap between the chip and the flat plate is filled with a thermal grease or gel. The grease or gel has sufficient compliance and viscosity to absorb the loads generated by the assembly process without transmitting those loads to the chips or interconnections of the package. The flat plate is generally attached to either a heat sink or a cold plate, in which a cooling fluid (e.g. air or water) is circulated to remove heat. This heat removal technique is simple but generally provides high heat removal capability. In order to increase the effectiveness of this cooling technique, it is generally desired to have a thermal grease or gel of high thermal conductivity and used in the smallest possible gap. An example of thermal grease suitable for use in such an arrangement and the invention is IBM Advanced Thermal Compound (ATC); the thermal conductivity of which is around 2.8 W/m.degree.K and can be placed in gaps ranging from 0.001 to 0.1 inches in single-chip or multi-chip modules.
It has been found, however, that thermal cycling (e.g. power on, power off cycling) tends to squeeze the thermal grease or gel out of the small gap it occupies between the chip and the flat plate. Thermal cycling causes the chip carrier to flex in the direction perpendicular to the flat plate surface which reduces the size of the gap occupied by the thermal grease or gel. Depending on the magnitude of the flexure and the thermal grease or gel properties, the spillage of thermal grease or gel out of the gap may leave some areas of the chip uncovered, leading to increased and irregular chip temperatures.
Temperature gradients developing in the substrate and flat plate during power on and power off and the ensuing thermal expansion of the substrate and flat plate tend to increase the squeezing of the thermal grease or gel and spillage thereof. Further, larger thermal gradients and larger squeezing forces result from increased power dissipation in the devices. The squeezing effect on the thermal grease or gel is aggravated when the squeezing motion becomes a proportionally larger fraction of the paste gap either as the paste gap is reduced by design or as uncovered areas of the chip increase; causing increase of thermal gradients and consequent flexure.
By increasing the paste gap to reduce the effect of the squeezing motion on the thermal grease or gel, the thermal performance of the module is degraded even with advanced thermal greases due to increased thermal resistance. Further, if the paste gap is increased, the thermal grease or gel may sag, especially under the influence of gravity or other acceleration forces in some orientations of the package. These effects are aggravated by higher temperatures which greatly reduce the viscosity of thermal greases or gels. It should be appreciated that both this effect, like the squeezing effect described above become greater over time as areas not covered by thermal grease increase, resulting in higher temperatures and thermal gradients within the package. Thus, while small paste gaps may be desirable to reduce the latter effect, small paste gaps can aggravate the formal effect and vice-versa, limiting the ability to control thermal performance over time.
It should also be recognized that other components such as decoupling capacitors may be mounted on modular circuit structures and may limit the degree to which small paste gaps can be accomplished. Further, larger paste gaps can provide substantial manufacturing economies when some reduction in thermal performance is tolerable. Therefore, regardless of the paste gap chosen for a particular design, known devices are subject to movement of paste and loss of thermal performance by one or both of the above-described mechanisms, although in differing relative degrees.
Given the cooling requirements have reached extreme levels in state-of-the-art modular circuit packages and designs, any loss of cooling capacity after a device is placed in service, particularly if not controlled or predictable, can lead to catastrophic failure and/or malfunction of the chips, module, any electronic device containing such a module or even a network to which the electronic device is connected. Accordingly, there is a need to limit movement of compliant thermal materials, regardless of the mechanism of such movement which does not transmit forces to the chips and which may thus cause damage thereto.